Diffusion and concentration gradients

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration because of random molecular motion. Particles move in both directions, but more initially leave the crowded region. At equilibrium, particles still move, although there is no net concentration change. Across a cell membrane, the substance must be able to cross, either through the membrane itself or through a suitable protein. Diffusion does not require direct metabolic energy input.